1. Field of Invention
The present invention relates generally to a liquid ink printing apparatus and a method for gray scale printing. More particularly, the invention relates to an ink jet printhead having different size drop ejectors.
2. Description of Related Art
Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead from which droplets of ink are ejected towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulse is usually produced by a heater transducer or resistor, typically associated with one of the channels. Each resistor is individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially bulges from the channel orifice followed by collapse of the bubble. The ink within the channel then retracts and separates from the bulging ink thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium whereupon hitting the recording medium a dot or spot of ink is deposited. The channel is then refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink.
An ink jet printhead can include one or more thermal ink jet printhead dies having an individual heater die and an individual channel die. The channel die includes an array of fluidic channels which bring ink into contact with the resistive heaters which are correspondingly arranged on the heater die. In addition, the die may also have integrated addressing electronics and driver transistors. Fabrication yields of die assemblies at a resolution on the order of 300-600 channels per inch is such that the number of channels per die is preferably in the range of 50-500 under current technology capabilities. Since the array of channels in a single die assembly is not sufficient to cover the length of a page, the printhead is either scanned across the page with a paper advance between scans or multiple die assemblies are butted together to produce a page width printbar. Because thermal ink jet nozzles typically produce spots or dots of a single size, high quality printing requires the fluidic channels and corresponding heaters to be fabricated at a high resolution on the order of 400-600 channels per inch.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge. The combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a stationary recording medium, such as paper or a transparency. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until the entire page is printed. In contrast, the page width printer includes a stationary printhead having a length sufficient to print across the width or length of a sheet of recording medium at a time. The recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. A page width ink-jet printer is described, for instance, in U.S. Pat. No. 5,192,959.
Printers typically print information received from an image output device such as a personal computer. Typically, this received information is in the form of a raster scan image such as a full page bitmap or in the form of an image written in a page description language. The raster scan image includes a series of scan lines consisting of bits representing pixel information in which each scan line contains information sufficient to print a single line of information across a page in a linear fashion. Printers can print bitmap information as received or can print an image written in the page description language once converted to a bitmap consisting of pixel information.
In a printer having a printhead with equally spaced nozzles, each of the same size nozzles producing ink spots of the same size, the pixels are placed on a square first grid having a size S, where S is generally the spacing between the marking transducers or channels on the printhead as illustrated in a sample printing pattern of FIG. 2. The nozzles 60 (schematically represented as triangles) traverse across a recording medium in the scan direction X as illustrated. The nozzles, which are spaced from one another a specified distance d, also known as the pitch, deposit ink spots or drops on pixel centers 62 on the grid having the grid spacing S in a direction perpendicular to the scanned direction, which is of course dependent on the spacing d. Typically, the nozzles and printing conditions are designed to produce spot diameters of approximately 1.414 (the square root of 2) times the grid spacing S. This allows complete filling of space, by letting diagonally adjacent pixels touch. A disadvantage of this printing scheme is that jaggedness may be objectionable at line edges, particularly for lines or curves at small angles to the scan direction as illustrated in FIG. 2. A first ellipse 64 located outside a second ellipse 66 in FIG. 2, indicate at what portions of the printed image the jaggedness would be most objectionable. In addition, print quality can be determined by 1) how much white space remains within the ring defined by the first and second ellipses, 2) how far the spots extend outside either the first or second ellipse, and 3) the amount of ink deposited on the recording medium.
One method of improving the line edge quality is to extend the addressability of the carriage to thereby allow dot placement at intermediate positions in the grid in the scanned direction. It is also possible to improve line edge quality by increasing the resolution. This, however, increases the complexity and cost of fabrication and typically slows down printing because of the additional number of spots to be printed.
The printheads and printing methods discussed above, and illustrated in FIG. 2 for example, provide for the printing of ink jet images having sufficient quality, especially when the resolution is increased upwards to 600 channels per inch. These printheads and methods, however, do not always provide images having the desired quality especially when considering gray scale levels, ink saving print modes, and printing throughput.
A majority of thermal ink jet printers produce spots or drops of ink all having the same diameter, within approximately about 10 percent, and are therefore not capable of gray scale printing. Drop volume or spot size is determined by many factors, including the heater transducer area, the cross sectional area of the ink ejecting channel or nozzle, the pulsing conditions necessary to create an ink droplet and the physical properties of the ink itself, such as the ink temperature. Although spot diameter changes of approximately xc2x110 percent are possible by changing pulsing conditions or ink temperature during printing, the given spot size is nominally constant to the extent that deliberate spot size variations cannot span a large enough range to be useful in gray scale printing.
Another method of improving printing quality, especially gray scale printing quality is to use groups of different size nozzles, as disclosed in U.S. Pat. No. 5,745,131 to Kneezel et al., which is hereby incorporated by reference into this disclosure. FIG. 3 illustrates printing according to U.S. Pat. No. 5,745,131 wherein a pattern is printed with a printhead having a first plurality of orifices 67 and a second plurality of orifices 68, producing spot diameters of 1.4S and 1.0S respectively. The spacing between nozzles of the first plurality of orifices 67 is again the distance d and the spacing between individual nozzles of the second plurality of orifices 68 is also the spacing d. The printing grid illustrated in FIG. 3 has a spacing of S between the pixel centers. The ink jet printer fires the individual nozzles of each plurality of orifices so that the ink drops land on the grid points in the scan direction. A somewhat better fill is achieved than previously illustrated in FIG. 2, at least in terms of the amount of ink used. Within the first ellipse 64 and the second ellipse 66, there are thirty-eight pixels of the large ink drops and sixteen pixels of the smaller ink drops which yields a more extensive coverage of ink within the first ellipse 64 and the second ellipse 66, even though the total amount of ink used is actually less than in FIG. 2. Since the number of nozzles within each of the first plurality of nozzles 67 and the second plurality of nozzles 68 are equivalent, the paper is advanced half the printhead length to achieve proper fill.
Various other methods and apparatus for gray scale printing with thermal ink jet printers and other ink jet printers include changing the ink drop size by either varying the driving signals to the transducer which generates the ink droplet or by creating a printhead which has a number of different sized ink ejecting orifices for creating gray scale images.
For example, U.S. Pat. No. 5,412,410 to Rezanka, discloses a printhead having different sized nozzles, which are alternated with each other according to size. As shown in FIG. 4, printhead 30 has large size nozzles 32 alternated with relatively small size nozzles 34 across the linear array. Each nozzle is spaced a distance S on center, with the large and small nozzles spaced apart by 2S, respectively. While gray scale printing can be effected by this arrangement, a large volume of ink is used and printing throughput or speed can be slow.
This invention addresses the above problems by providing methods of printing with a printhead having different size nozzles to effectively and efficiently fill spaces between pixels.
According to one aspect of the invention, a method of printing by ejecting droplets of ink onto a printing substrate with a printhead having a plurality of drop ejectors with different sizes, comprises the steps of firing a first size drop ejector in one printing pass, shifting the printhead with respect to the printing substrate, and firing a second size drop ejector in a second printing pass so that the ejected droplets form spots having different diameters that are aligned in the printing direction.
According to another aspect of the invention, a method of firing drop ejectors by ejecting droplets of ink onto a printing substrate with a printhead having a plurality of drop ejectors with different sizes, comprises the steps of tilting the printhead from vertical with respect to the scanning direction, firing a first size drop ejector in one printing pass, and firing a second size drop ejector in a second printing pass so that the ejected droplets form spots having different diameters that are offset in the direction perpendicular to the scanning direction.
According to a further aspect of the invention, a method of printing ink spots on a printing medium using at least two different size nozzles in a printhead array including a plurality of longitudinally extending channels, each channel having a central longitudinal axis and an end forming a nozzle for ejecting ink drops, wherein each nozzle is offset from the longitudinal axis of the associated channel and adjacent nozzles are spaced apart in opposing sides of the adjacent channels to achieve maximum spacing between adjacent nozzles on one side and minimum spacing between nozzles on another side, comprises the steps of firing one size nozzle in one printing pass, advancing the printing medium an odd number of pixels, and firing another size nozzle in another printing pass, wherein the different size nozzles are fired in a staggered fashion so that one size spot is displaced xc2xd pixel in both directions relative to the other size spot.